WO2015172485A1 - Apparatus and method for detecting narrow groove of specular reflection workpiece - Google Patents

Apparatus and method for detecting narrow groove of specular reflection workpiece Download PDF

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Publication number
WO2015172485A1
WO2015172485A1 PCT/CN2014/086847 CN2014086847W WO2015172485A1 WO 2015172485 A1 WO2015172485 A1 WO 2015172485A1 CN 2014086847 W CN2014086847 W CN 2014086847W WO 2015172485 A1 WO2015172485 A1 WO 2015172485A1
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Prior art keywords
imaging unit
workpiece surface
laser
image
coordinate system
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PCT/CN2014/086847
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French (fr)
Inventor
Dong Du
Jinle ZENG
Yirong ZOU
Guoqing Wang
Jiluan Pan
Li Wang
Baohua CHANG
Wenzeng ZHANG
Zandong HAN
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Tsinghua University
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Priority to JP2015563076A priority Critical patent/JP6101370B2/en
Publication of WO2015172485A1 publication Critical patent/WO2015172485A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K9/00—Arc welding or cutting
    • B23K9/12—Automatic feeding or moving of electrodes or work for spot or seam welding or cutting
    • B23K9/127—Means for tracking lines during arc welding or cutting
    • B23K9/1272—Geometry oriented, e.g. beam optical trading
    • B23K9/1274—Using non-contact, optical means, e.g. laser means
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K9/00—Arc welding or cutting
    • B23K9/02—Seam welding; Backing means; Inserts
    • B23K9/0213—Narrow gap welding

Definitions

  • Embodiments of the present disclosure generally relate to a welding automation field, and more particularly, to an apparatus and a method for detecting a narrow groove of a specular reflection workpiece.
  • a groove of a workpiece to be welded is generally a square groove, and a groove gap thereof is very small (generally no more than 0.1mm) and a serious welding defect may be caused due to a slight deviation between a welding torch and the groove, thus requiring a high accuracy for the detecting and tracking.
  • a material of most aeronautics and astronautics members is an aluminium-magnesium alloy having a reflectivity more than 95%, and an image brightness thereof is nonuniform due to a strong specular reflection light from a surface of the aluminium-magnesium alloy, and even main feature information of the groove may be missed.
  • the conventional method for tracking the welding seam identifies a welding area by detecting a distortion characteristic of a structured light stripe. The method, however, is overly dependent on a macroscopic geometric feature of the groove and thus it is difficult to apply the method to a narrow groove without obvious distortion characteristics of the structured light stripe.
  • the Chinese Patent No. CN101927395B discloses an apparatus and a method for tracking and detecting a welding seam, in which a laser spot having a specific profile feature is projected onto a workpiece surface, and an image of the workpiece surface is sampled by a CCD camera. By detecting a groove shadow within the laser spot, a transversal deviation of the groove can be obtained, and by detecting a shape, a position and a size of the laser spot, a relative pose between the workpiece surface and the welding torch can be calculated.
  • the image sampled by this method has a nonuniform gray scale, which causes difficulties on accurately extracting an edge of the laser spot. This disadvantage is caused by two reasons.
  • Embodiments of the present disclosure seek to solve at least one of the problems existing in the related art to at least some extent.
  • Embodiments of a first broad aspect of the present disclosure provide an apparatus for detecting a narrow groove of a specular reflection workpiece.
  • the apparatus includes: a spherical light source having a semispherical diffuse reflection body and a light emitting diode array arranged over the semispherical diffuse reflection body, and configured to generate a uniform illumination on a workpiece surface; a laser array having at least three lasers and configured to emit a laser light onto the workpiece surface; an imaging unit configured to sample a light reflected from the workpiece surface and emitted from the spherical light source or the laser array so as to generate an image of the workpiece surface; and a control unit electrically connected with the spherical light source, the laser array and the imaging unit respectively, and configured to control the spherical light source and the laser array to illuminate the workpiece surface alternately, to control the imaging unit to sample a first image of the workpiece surface illuminated by the laser array and a second image of the workpiece surface illuminated by the
  • the spherical light source further includes an aperture, through which the laser light emitted by the laser array reaches the workpiece surface.
  • the light reflected from the workpiece surface and emitted from the spherical light source or the laser array reaches the imaging unit through the aperture.
  • the apparatus further includes a light filtering unit arranged in a light path between the spherical light source and the imaging unit and configured to filter an arc light generated during a welding process and an ambient light.
  • the light emitting diode array is arranged on a bottom of the semispherical diffuse reflection body and the light emitted by the light emitting diode array reaches the workpiece surface after being reflected by the semispherical diffuse reflection body.
  • the light emitting diode array arranged on an inner surface of the semispherical diffuse reflection body, and a part of the light emitted by the light emitting diode array reaches the workpiece surface directly and another part of the light emitted by the light emitting diode array reaches the workpiece surface after being reflected by the semispherical diffuse reflection body.
  • control unit is further configured to calculate a normal vector of the workpiece surface according to the first image.
  • the pose of the welding torch with respect to the workpiece surface can be adjusted automatically according to the position of the center point of the narrow groove and the normal vector of the workpiece surface.
  • the imaging unit comprises any one of a charge-coupled device, a complementary metal oxide semiconductor imaging device, a position sensitive device and a charge-injection device.
  • a wavelength of the light emitted by the spherical light source or the laser array is consistent with a central wavelength of the light filtering unit, and the central wavelength of the light filtering unit is within a sensitive wavelength range of the imaging unit.
  • the apparatus further includes a housing, in which the welding torch is fixedly connected to the housing, and the spherical light source, the laser array, the imaging unit and the light filtering unit are fixedly arranged in the housing respectively.
  • the workpiece surface is illuminated by the spherical light source to obtain a position deviation of the groove and the pose information of the workpiece surface is determined by the laser array.
  • a gray scale of the image is uniform and groove features are clear, such that it is convenient to detect a position of the narrow groove accurately and in real time;
  • the pose information of the welding torch with respect to the workpiece surface can be determined quickly and accurately, in which the pose information includes a lateral deviation, an altitude-direction deviation, a lateral deflection angle and a longitudinal deflection angle of the welding torch;
  • a detection accuracy can be improved as high as 0.03mm;
  • the apparatus has a simple structure, a low cost and a good real-time performance, and applies to an automatic detection of the groove of the strong specular reflection workpiece, and particularly to a detection of a narrow groove having a groove gap less than 0.1mm.
  • Embodiments of a second broad aspect of the present disclosure provide a method for detecting a narrow groove of a specular reflection workpiece, including:
  • an origin of the imaging unit coordinate system ⁇ C ⁇ is an optical center of an imaging unit and a direction of a vertical axis of the imaging unit coordinate system ⁇ C ⁇ is identical to that of an optical axis of the imaging unit;
  • N is an integer larger than or equal to 3;
  • a i [X i,0 +t i,1 (u i , v i ) ⁇ S i +t i,2 (u i , v i ) ⁇ V i (u i , v i ) ] /2
  • V i (u i , v i ) [f 1 (u i , v i ) , f 2 (u i , v i ) , 1] T
  • V w (u w , v w ) [f 1 (u w , v w ) , f 2 (u w , v w ) , 1] T .
  • the coordinate (u w , v w ) T of the center point of the narrow groove in the pixel coordinate system ⁇ P ⁇ is denoted as:
  • a pose of a welding torch with respect to the workpiece can be adjusted automatically according to the coordinate B and the normal vector ⁇ of the workpiece surface.
  • the workpiece surface is illuminated by the spherical light source to obtain a position deviation of the groove and the pose information of the workpiece surface is determined by the laser array.
  • a gray scale of the image is uniform and groove features are clear, such that it is convenient to detect a position of the narrow groove accurately and in real time; the pose information of the welding torch with respect to the workpiece surface can be determined quickly and accurately, in which the pose information includes a lateral deviation, an altitude-direction deviation, a lateral deflection angle and a longitudinal deflection angle of the welding torch; a detection accuracy can be improved as high as 0.03mm.
  • Fig. 1 is a schematic structural view of an apparatus for detecting a narrow groove of a specular reflection workpiece according to Embodiment 1 of the present disclosure
  • Fig. 2 is a schematic structural view of an apparatus for detecting a narrow groove of a specular reflection workpiece according to Embodiment 2 of the present disclosure
  • Fig. 3 is an image of a groove sampled by an imaging unit when a spherical light source illuminates a workpiece according to Embodiment 1 and Embodiment 2 of the present disclosure;
  • Fig. 4 is a schematic view showing determining a pose of a workpiece by a laser array according to Embodiment 1 and Embodiment 2 of the present disclosure.
  • Fig. 5 is a flow chart of a method for detecting a narrow groove of a specular reflection workpiece according to embodiments of the present disclosure.
  • relative terms such as “central” , “longitudinal” , “lateral” , “front” , “rear” , “right” , “left” , “inner” , “outer” , “lower” , “upper” , “horizontal” , “vertical” , “above” , “below” , “up” , “top” , “bottom” as well as derivative thereof (e.g. , “horizontally” , “downwardly” , “upwardly” , etc. ) should be construed to refer to the orientation as then described or as shown in the drawings under discussion. These relative terms are for convenience of description and do not require that the present disclosure be constructed or operated in a particular orientation.
  • the terms “mounted, ” “connected, ” and “coupled” and variations thereof are used broadly and encompass such as mechanical or electrical mountings, connections and couplings, also can be inner mountings, connections and couplings of two components, and further can be direct and indirect mountings, connections, and couplings, which can be understood by those skilled in the art according to the detail embodiment of the present disclosure.
  • first and second are used herein for purposes of description and are not intended to indicate or imply relative importance or significance or to imply the number of indicated technical features.
  • the feature defined with “first” and “second” may comprise one or more of this feature.
  • “aplurality of” means two or more than two, unless specified otherwise.
  • Fig. 1 is a schematic structural view of an apparatus for detecting a narrow groove of a specular reflection workpiece according to Embodiment 1 of the present disclosure.
  • the apparatus includes a control unit 1, a spherical light source 3, a laser array 4 and an imaging unit 5.
  • Each of the spherical light source 3, the laser array 4 and the imaging unit 5 has a fixed relative position with respect to a welding torch 8.
  • the spherical light source 3 has a semispherical diffuse reflection body 32 and a light emitting diode array 31 arranged over the semispherical diffuse reflection body 32 and is configured to generate a uniform illumination on a workpiece surface 7.
  • the laser array 4 has at least three lasers and is configured to emit a laser light onto the workpiece surface 7 so as to determine a pose of the welding torch 8 with respect to the workpiece surface 7.
  • the imaging unit 5 is configured to sample a light reflected from the workpiece surface 7 and emitted from the spherical light source 3 or the laser array 4 so as to generate an image of the workpiece surface 7.
  • the control unit 1 is electrically connected with the spherical light source 3, the laser array 4 and the imaging unit 5 respectively, and is configured to control the spherical light source 3 and the laser array 4 to illuminate the workpiece surface 7 alternately, to control the imaging unit 5 to sample a first image of the workpiece surface 7 illuminated by the laser array 4 and a second image of the workpiece surface illuminated by the spherical light source 3, to calculate the pose of the welding torch 8 with respect to the workpiece according to the first image, and to calculate a position of a center point of the narrow groove according to the second image and the pose of the welding torch 8 with respect to the workpiece.
  • the apparatus further includes a light filtering unit 6.
  • the light filtering unit 6 is arranged in a light path between the spherical light source 3 and the imaging unit 5 and is configured to filter an arc light generated during a welding process and an ambient light.
  • control unit 1 may be connected with the spherical light source 3, the laser array 4 and the imaging unit 5 via a wire.
  • the spherical light source 3 further includes an aperture 33, through which the laser light emitted by the laser array 4 reaches the workpiece surface 7.
  • the light emitting diode array 31 is arranged on a bottom of the semispherical diffuse reflection body 32 and the light emitted by the light emitting diode array 31 reaches the workpiece surface 7 after being reflected by the semispherical diffuse reflection body 32.
  • a laser spot emitted from the laser array 4 is also projected onto the workpiece surface 7 and a light reflected from the workpiece surface 7 reaches the imaging unit 5 after passing through the aperture 33 and the light fileting unit 6.
  • the light reflected from the workpiece surface 7 and emitted from the spherical light source 3 or the laser array 4 reaches the imaging unit 5 through the aperture 33.
  • the light emitting diode array 31 may provide an incoherent light having a uniform light density and without a disadvantage of speckle.
  • the imaging unit 5 includes, but not limited to, any one of a charge-coupled device, a complementary metal oxide semiconductor imaging device, a position sensitive device and a charge-injection device.
  • a wavelength of the light emitted by the spherical light source 3 or the laser array 4 is consistent with a central wavelength of the light filtering unit 6, and the central wavelength of the light filtering unit 6 is within a sensitive wavelength range of the imaging unit 5.
  • the number of the lasers is three, namely a first laser 41, a second laser 42 and a third laser 43 and a wavelength thereof is 635mm.
  • the imaging unit 5 is a 1024 ⁇ 1024 CCD camera, and a field range thereof is 30mm ⁇ 30mm and a detection accuracy thereof is 0.03mm.
  • a wavelength of the light emitted by the light emitting dioxide array 31 ranges from 635nm to 645nm.
  • the light filtering unit 6 is a narrow bandpass filter whose central wavelength is 635nm and full width at half maximum is 10nm. Since the arc light has the relative weak light density at the wavelength range of 635nm-645nm, the light filtering unit 6 can filter the arc light generated during the welding process.
  • the apparatus further includes a housing 2.
  • the welding torch 8 is fixedly connected to the housing 2, and the spherical light source 3, the laser array 4, the imaging unit 5 and the light filtering unit 6 are fixedly arranged in the housing 2 respectively.
  • Fig. 2 is a schematic structural view of an apparatus for detecting a narrow groove of a specular reflection workpiece according to Embodiment 2 of the present disclosure.
  • Embodiment 2 differs from Embodiment 1 in that the light emitting dioxide array 31 is arranged on the inner surface of the semispherical diffuse reflection body 32.
  • the light emitting dioxide array 31 is arranged on the inner surface of the semispherical diffuse reflection body 32.
  • a part of the light emitted by the light emitting diode array 31 reaches the workpiece surface 7 directly and another part of the light emitted by the light emitting diode array 31 reaches the workpiece surface 7 after being reflected by the semispherical diffuse reflection body 32.
  • the light density projected onto the workpiece surface 7 is uniform due to the light intensity integration effect of the semispherical diffuse reflection body 32.
  • Fig. 3 is an image of a groove sampled by an imaging unit when a spherical light source illuminates a workpiece according to Embodiment 1 and Embodiment 2 of the present disclosure.
  • a base metal 71 and the groove 72 have great different optical reflection characteristics.
  • a gray scale of the base metal 71 is nearly saturated due to a strong specular reflection on a surface of the base metal 71.
  • the groove 72 is represented as a curve in the image, in which a gray scale of the groove 72 is close to zero, since the light projected onto the groove 72 is reflected by a side wall of the groove 72, and thus not reaching the imaging unit 5. Because of the great difference between the gray scales of the base metal 71 and the groove 72, the position of the groove 72 may be detected quickly and accurately.
  • Fig. 4 is a schematic view showing determining a pose of a workpiece by a laser array according to Embodiment 1 and Embodiment 2 of the present disclosure.
  • the laser array 4 is used to determine the pose of the welding torch 8 with respect to the workpiece surface 7 and a pixel coordinate system ⁇ P ⁇ is established in the image sampled by the imaging unit 5, in which any point in the pixel coordinate system ⁇ P ⁇ represents a pixel coordinate of the image sampled by the imaging unit 5.
  • an imaging unit coordinate system ⁇ C ⁇ is established, in which an origin of the imaging unit coordinate system ⁇ C ⁇ is an optical center of the imaging unit 5 and a direction of a vertical axis of the imaging unit coordinate system ⁇ C ⁇ is identical to that of an optical axis of the imaging unit 5.
  • Fig. 5 is a flow chart of a method for detecting a narrow groove of a specular reflection workpiece according to embodiments of the present disclosure.
  • the imaging unit 5 samples an image of the workpiece surface 7, i.e. , the imaging unit 5 samples the first image of a workpiece surface 7 illuminated by the laser array 4, so as to calculate the pose of the welding torch 8 with respect to the workpiece surface 7. Furthermore, the control unit 1 can also calculate a normal vector of the workpiece surface 7 according to the first image.
  • the controls unit 1 controls the spherical light source to turn on and the laser array 4 to turn off
  • the imaging unit 5 samples an image of the workpiece surface 7, i.e.
  • the imaging unit 5 samples the second image of the workpiece surface 7 illuminated by the spherical light source 3, such that a coordinate of a center point of the narrow groove 72 in the imaging unit coordinate system ⁇ C ⁇ can be calculated according to a pixel coordinate of the center point of the narrow groove 72 in the second image and a partial calculation result of the pose of the welding torch 8 with respect to the workpiece surface 7 (i.e. , the normal vector of the workpiece surface 7) .
  • the control unit 1 is configured to control the spherical light source 3, the laser array 4 and the imaging unit 5, to process the images sampled by the imaging unit 5 and to automatically adjust the pose of the welding torch 8 with respect to the workpiece surface 7, thus implementing an automatic tracking.
  • a method for detecting a narrow groove of a specular reflection workpiece includes following steps.
  • An imaging unit coordinate system ⁇ C ⁇ is established, in which an origin of the imaging unit coordinate system ⁇ C ⁇ is an optical center of an imaging unit and a direction of a vertical axis of the imaging unit coordinate system ⁇ C ⁇ is identical to that of an optical axis of the imaging unit, and a pixel coordinate system ⁇ P ⁇ is established.
  • Equation (1) a propagation path equation of a laser light emitted by an i th laser in the imaging unit coordinate system ⁇ C ⁇ can be obtained, which is denoted as Equation (1) :
  • i is an integer larger than or equal to 1 and less than or equal to N;
  • X i and X i,0 are points in a propagation path of the laser light emitted by the i th laser;
  • S i is a unit direction vector of the propagation path of the laser light emitted by the i th laser;
  • t i is a directed distance between the points X i and X i,0 .
  • the imaging unit is calibrated (for example, by a Zhang Zhengyou calibrating method) to obtain X i,0 and S i in Equation (1) and a transformation relationship between a point (u, v) T in the pixel coordinate system ⁇ P ⁇ and a corresponding point (x, y, z) T in the imaging unit coordinate system ⁇ C ⁇ , which is denoted as Equation (2) :
  • f 1 and f 2 are transformation functions between the pixel coordinate system ⁇ P ⁇ and the imaging unit coordinate system ⁇ C ⁇ ;
  • the imaging unit samples a first image of a workpiece surface illuminated by the laser array. Then, the first image is processed to calculate a coordinate (u i , v i ) T of a laser spot of the i th laser in the pixel coordinate system ⁇ P ⁇ and to calculate a coordinate A i of the laser spot of the i th laser in imaging unit coordinate system ⁇ C ⁇ according to the coordinate (u i , v i ) T .
  • the imaging unit samples the first image of the workpiece surface illuminated by the laser array.
  • a pixel coordinate of the laser spot projected onto the workpiece surface 7 and emitted from the i th laser in the first image sampled by the imaging unit is (u i , v i ) T
  • V i (u i , v i ) [f 1 (u i , v i ) , f 2 (u i , v i ) , 1] T
  • t i,2 (u i , v i ) is an undetermined parameter depending on u i and v i . It is indicated in Equation (3) that the point A i is in a straight line represented by Equation (3) , and the straight line passes the origin of the imaging unit coordinate system ⁇ C ⁇ and defines a direction vector V i (u i , v i ) .
  • Equation (1) Since the point A i is in a straight line represented by Equation (1) , the point A i may also be represented by Equation (4) :
  • a i X i,0 +t i,1 (u i , v i ) S i (4)
  • t i,1 (u i , v i ) is an undetermined parameter depending on u i and v i .
  • the point A i is an intersection point of two straight lines represented by Equation (3) and Equation (4) , and the vectors S i and V i (u i , v i ) are not in parallel with each other, otherwise the point A i will not exist.
  • the straight lines represented by Equation (3) and Equation (4) are generally skewed lines due to a measurement error and an interference noise, and thus the point A i is taken as a midpoint of a common perpendicular of the two skewed lines. Since a common perpendicular segment of the two skewed lines is a shortest one of line segments connecting two points respective in the two skewed lines, an objective function is established as:
  • t i,1 (u i , v i ) and t i,2 (u i , v i ) can be determined by solving a minimum value of the objective function g, such that the coordinate of the point A i can be obtained.
  • the objective function g is processed as follows:
  • a determinant of coefficient of equation set (7) is:
  • ⁇ S i , V i (u i , v i ) > represents an included angle of the vectors S i and V i (u i , v i ) .
  • equation set (7) Since the vectors S i and V i (u i , v i ) are not in parallel with each other, a determinant of equation (8) is larger than zero and equation set (7) has a unique solution which is denoted as:
  • a i [X i,0 +t i,1 (u i , v i ) ⁇ S i +t i,2 (u i , v i ) ⁇ V i (u i , v i ) ] /2 (11)
  • Equation (11) the coordinate A i of the laser spot projected onto the workpiece surface and emitted from the i th laser in the imaging unit coordinate system ⁇ C ⁇ is given by Equation (11) .
  • W the workpiece surface onto which the laser spot is projected.
  • the laser array is required to include at least three lasers.
  • the imaging unit samples a second image of the workpiece surface illuminated by the spherical light source.
  • a filtering and threshold segmentation process is performed on the second image, a coordinate (u w , v w ) T of a center point of the narrow groove in the pixel coordinate system ⁇ P ⁇ is extracted, and a coordinate B of the center point of the narrow groove in the imaging unit coordinate system ⁇ C ⁇ is calculated according to the coordinate (u w , v w ) T .
  • the imaging unit samples a gray scale image of the workpiece surface, i.e. , the second image.
  • the filtering and threshold segmentation process is performed on the second image sampled by the imaging unit so as to obtain a binary image I.
  • the gray scale of a base metal is 1
  • the gray scale of the narrow groove is 0.
  • the coordinate (u w , v w ) T of the center point of the narrow groove in the pixel coordinate system ⁇ P ⁇ is denoted as:
  • I (j, v) represents a gray value of a pixel point in a j th row and a v th column of the image I;
  • the coordinate B of the center point of the narrow groove in the imaging unit coordinate system ⁇ C ⁇ can be calculated according to equations (16) and (17) ,i.e.,
  • a pose of a welding torch with respect to the workpiece surface can be adjust automatically according to the coordinate B of the center point of the narrow groove and the normal vector ⁇ of the workpiece surface, thus implementing an automatic identification and tracking of the narrow groove.
  • the laser array may include more than three lasers so as to improve the detection accuracy of the pose of the welding torch with respect to the workpiece surface; an imaging unit with a higher resolution may be used herein to improve the detection accuracy of the groove; and the light filtering unit may be replaced by a spectrometer (such as a monochrometer) .
  • the present disclosure uses the spherical light source, the laser array and the imaging unit to implement the detection of the narrow groove of the specular reflection workpiece.
  • the apparatus and the method do not depend on macroscopic geometric structure properties of the groove and the detection accuracy is improved as high as 0.03mm.
  • the spherical light source is used to generate the uniform illumination on the workpiece surface, such that the gray scale of the base metal is close to 1 and the gray scale of the groove is close to 0. Therefore, on one hand, the position of the groove in the image can be obtained accurately; on the other hand, a difficulty of processing the image and an algorithm complexity are reduced.
  • the laser array is used to determine the pose information of the workpiece surface, thus simplifying the method and calculating the pose deviation of the welding torch with respect to the workpiece surface quickly and accurately.
  • the light filtering unit is used to filter the interference of the arc light and the ambient light on the imaging unit, such that the adaptability of the apparatus to an actual welding operation environment can be improved.
  • the apparatus has a simple structure, a low cost and a good real-time performance, and applies to an automatic detection of the groove of the strong specular reflection workpiece, and particularly to a detection of the narrow groove having a groove gap less than 0.1mm.

Abstract

An apparatus and a method for detecting a narrow groove (72) of a specular reflection workpiece are provided. The apparatus includes: a spherical light source (3) configured to generate a uniform illumination on a workpiece surface (7); a laser array (4) having at least three lasers and configured to emit a laser light onto the workpiece surface (7); an imaging unit (5) configured to sample a light from the workpiece surface (7) to generate an image thereof; and a control unit (1) configured to control the spherical light source (3) and the laser array (4) to illuminate the workpiece surface (7) alternately, to control the imaging unit (5) to sample the image of the illuminated workpiece surface (7) respectively, to calculate a pose of a welding torch (8) with respect to the workpiece and a position of a center point of the groove (72) according to the image.

Description

APPARATUS AND METHOD FOR DETECTING NARROW GROOVE OF SPECULAR REFLECTION WORKPIECE
CROSS REFERENCE TO RELATED APPLICATION
This application claims priority and benefits of Chinese Patent Application No. 201410201151.3, filed with State Intellectual Property Office on May 13, 2014, the entire content of which is incorporated herein by reference.
FIELD
Embodiments of the present disclosure generally relate to a welding automation field, and more particularly, to an apparatus and a method for detecting a narrow groove of a specular reflection workpiece.
BACKGROUND
Developments of lightening and improvements of reliability requirement on aeronautics and astronautics member bring a great challenge to a welding detecting and tracking. On one hand, a groove of a workpiece to be welded is generally a square groove, and a groove gap thereof is very small (generally no more than 0.1mm) and a serious welding defect may be caused due to a slight deviation between a welding torch and the groove, thus requiring a high accuracy for the detecting and tracking. On the other hand, a material of most aeronautics and astronautics members is an aluminium-magnesium alloy having a reflectivity more than 95％, and an image brightness thereof is nonuniform due to a strong specular reflection light from a surface of the aluminium-magnesium alloy, and even main feature information of the groove may be missed. The conventional method for tracking the welding seam identifies a welding area by detecting a distortion characteristic of a structured light stripe. The method, however, is overly dependent on a macroscopic geometric feature of the groove and thus it is difficult to apply the method to a narrow groove without obvious distortion characteristics of the structured light stripe.
The Chinese Patent No. CN101927395B discloses an apparatus and a method for tracking and detecting a welding seam, in which a laser spot having a specific profile feature is projected onto a workpiece surface, and an image of the workpiece surface is sampled by a CCD camera. By  detecting a groove shadow within the laser spot, a transversal deviation of the groove can be obtained, and by detecting a shape, a position and a size of the laser spot, a relative pose between the workpiece surface and the welding torch can be calculated. The image sampled by this method has a nonuniform gray scale, which causes difficulties on accurately extracting an edge of the laser spot. This disadvantage is caused by two reasons. On one hand, parts of the image are saturated due to a strong specular reflection on the laser by the workpiece surface. On the other hand, speckles are formed on the workpiece surface because of the laser irradiation, which grows the nonuniformity of the gray scale of the sampled image. Even though effects of the specular reflection light can be reduced to a certain extent by decreasing an exposure time, reducing an aperture or using a polaroid for extinction, the laser speckle phenomenon is more obvious and a uniformity of the gray scale of the image cannot be improved.
SUMMARY
Embodiments of the present disclosure seek to solve at least one of the problems existing in the related art to at least some extent.
Embodiments of a first broad aspect of the present disclosure provide an apparatus for detecting a narrow groove of a specular reflection workpiece. The apparatus includes: a spherical light source having a semispherical diffuse reflection body and a light emitting diode array arranged over the semispherical diffuse reflection body, and configured to generate a uniform illumination on a workpiece surface； a laser array having at least three lasers and configured to emit a laser light onto the workpiece surface； an imaging unit configured to sample a light reflected from the workpiece surface and emitted from the spherical light source or the laser array so as to generate an image of the workpiece surface； and a control unit electrically connected with the spherical light source, the laser array and the imaging unit respectively, and configured to control the spherical light source and the laser array to illuminate the workpiece surface alternately, to control the imaging unit to sample a first image of the workpiece surface illuminated by the laser array and a second image of the workpiece surface illuminated by the spherical light source, to calculate a pose of a welding torch with respect to the workpiece according to the first image, and to calculate a position of a center point of the narrow groove according to the second image and the pose of the welding torch with respect to the workpiece. 
In some embodiments, the spherical light source further includes an aperture, through which the laser light emitted by the laser array reaches the workpiece surface.
In some embodiments, the light reflected from the workpiece surface and emitted from the spherical light source or the laser array reaches the imaging unit through the aperture.
In some embodiments, the apparatus further includes a light filtering unit arranged in a light path between the spherical light source and the imaging unit and configured to filter an arc light generated during a welding process and an ambient light.
In some embodiments, the light emitting diode array is arranged on a bottom of the semispherical diffuse reflection body and the light emitted by the light emitting diode array reaches the workpiece surface after being reflected by the semispherical diffuse reflection body.
In some embodiments, the light emitting diode array arranged on an inner surface of the semispherical diffuse reflection body, and a part of the light emitted by the light emitting diode array reaches the workpiece surface directly and another part of the light emitted by the light emitting diode array reaches the workpiece surface after being reflected by the semispherical diffuse reflection body.
In some embodiments, the control unit is further configured to calculate a normal vector of the workpiece surface according to the first image.
In some embodiments, the pose of the welding torch with respect to the workpiece surface can be adjusted automatically according to the position of the center point of the narrow groove and the normal vector of the workpiece surface.
In some embodiments, the imaging unit comprises any one of a charge-coupled device, a complementary metal oxide semiconductor imaging device, a position sensitive device and a charge-injection device.
In some embodiments, a wavelength of the light emitted by the spherical light source or the laser array is consistent with a central wavelength of the light filtering unit, and the central wavelength of the light filtering unit is within a sensitive wavelength range of the imaging unit.
In some embodiments, the apparatus further includes a housing, in which the welding torch is fixedly connected to the housing, and the spherical light source, the laser array, the imaging unit and the light filtering unit are fixedly arranged in the housing respectively.
With the apparatus for detecting a narrow groove of a specular reflection workpiece  according to embodiments of the present disclosure, the workpiece surface is illuminated by the spherical light source to obtain a position deviation of the groove and the pose information of the workpiece surface is determined by the laser array. The apparatus according to embodiments of the present disclosure have following advantages: a gray scale of the image is uniform and groove features are clear, such that it is convenient to detect a position of the narrow groove accurately and in real time； the pose information of the welding torch with respect to the workpiece surface can be determined quickly and accurately, in which the pose information includes a lateral deviation, an altitude-direction deviation, a lateral deflection angle and a longitudinal deflection angle of the welding torch； a detection accuracy can be improved as high as 0.03mm； the apparatus has a simple structure, a low cost and a good real-time performance, and applies to an automatic detection of the groove of the strong specular reflection workpiece, and particularly to a detection of a narrow groove having a groove gap less than 0.1mm.
Embodiments of a second broad aspect of the present disclosure provide a method for detecting a narrow groove of a specular reflection workpiece, including:
establishing an imaging unit coordinate system {C} , in which an origin of the imaging unit coordinate system {C} is an optical center of an imaging unit and a direction of a vertical axis of the imaging unit coordinate system {C} is identical to that of an optical axis of the imaging unit；
establishing a pixel coordinate system {P} ；
assuming that a laser array has N lasers, in which N is an integer larger than or equal to 3；
calibrating the imaging unit to obtain a transformation relationship between a point (u, v)T in the pixel coordinate system {P} and a corresponding point (x, y, z)T in the imaging unit coordinate system {C} :
Figure PCTCN2014086847-appb-000001
and to obtain a propagation path equation of a laser light emitted by an ith laser in the imaging unit coordinate system {C} :
Xi＝Xi,0+tiSi
where f1 and f2 are transformation functions between the pixel coordinate system {P} and the imaging unit coordinate system {C} , i is an integer larger than or equal to 1 and less than or equal to N； Xi and Xi,0 are points in a propagation path of the laser light emitted by the ith laser； Si is a  unit direction vector of the propagation path of the laser light emitted by the ith laser； ti is a directed distance between the points Xi and Xi,0；
sampling a first image of a workpiece surface illuminated by the laser array via the imaging unit；
processing the first image, calculating a coordinate (ui, vi)T of a laser spot of the ith laser in the pixel coordinate system {P} and calculating a coordinate Ai of the laser spot of the ith laser in imaging unit coordinate system {C} according to the coordinate (ui, vi) T:
Ai＝ [Xi,0+ti,1 (ui, vi) · Si+ti,2 (ui, vi) · Vi (ui, vi) ] /2
where
Vi(ui, vi) ＝ [f1 (ui, vi) , f2 (ui, vi) , 1]T
Figure PCTCN2014086847-appb-000002
Figure PCTCN2014086847-appb-000003
assuming that the workpiece surface where the laser spot is projected to a plane W, an equation of the plane W is denoted as XTα＝1, where α is a normal vector of the plane W, and X is a point in the plane W, provided the point Ai is in the plane W, there being:
Figure PCTCN2014086847-appb-000004
namely：
Figure PCTCN2014086847-appb-000005
solving the normal vector α of the plane W；
sampling a second image of the workpiece surface illuminated by the spherical light source via the imaging unit；
performing a filtering and threshold segmentation process on the second image, extracting a coordinate (uw, vw)T of a center point of the narrow groove in the pixel coordinate system {P} , and calculating a coordinate B of the center point of the narrow groove in the imaging unit coordinate system {C} according to the coordinate (uw, vw)T:
B＝Vw (uw, vw)/[αTVw (uw, vw) ]
where
Vw(uw, vw) ＝ [f1 (uw, vw) , f2 (uw, vw) , 1]T.
In some embodiments, the coordinate (uw, vw)T of the center point of the narrow groove in the pixel coordinate system {P} is denoted as:
uw＝j
Figure PCTCN2014086847-appb-000006
where I (j, v) represents a gray value of a pixel point in a jth row and a vth column of an image I； # {v: I(j, v) ＝ 0} represents a sum of pixel points satisfying the equation: I (j, v) ＝ 0； and sum {v: I (j, v) ＝ 0} represents a sum of column numbers of pixel points satisfying the equation: I (j, v) ＝ 0.
In some embodiments, a pose of a welding torch with respect to the workpiece can be adjusted automatically according to the coordinate B and the normal vector α of the workpiece surface.
With the method for detecting a narrow groove of a specular reflection workpiece according to embodiments of the present disclosure, the workpiece surface is illuminated by the spherical light source to obtain a position deviation of the groove and the pose information of the workpiece surface is determined by the laser array. The method according to embodiments of the present disclosure have following advantages: a gray scale of the image is uniform and groove features are clear, such that it is convenient to detect a position of the narrow groove accurately and in real time； the pose information of the welding torch with respect to the workpiece surface can be determined quickly and accurately, in which the pose information includes a lateral deviation, an altitude-direction deviation, a lateral deflection angle and a longitudinal deflection angle of the welding torch； a detection accuracy can be improved as high as 0.03mm.
The above summary of the present disclosure is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The figures and the detailed description which follow more particularly exemplify illustrative embodiments.
Additional aspects and advantages of embodiments of present disclosure will be given in part in the following descriptions, become apparent in part from the following descriptions, or be learned from the practice of the embodiments of the present disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects and advantages of embodiments of the present disclosure will become apparent and more readily appreciated from the following descriptions made with reference to the accompanying drawings, in which:
Fig. 1 is a schematic structural view of an apparatus for detecting a narrow groove of a specular reflection workpiece according to Embodiment 1 of the present disclosure；
Fig. 2 is a schematic structural view of an apparatus for detecting a narrow groove of a specular reflection workpiece according to Embodiment 2 of the present disclosure；
Fig. 3 is an image of a groove sampled by an imaging unit when a spherical light source illuminates a workpiece according to Embodiment 1 and Embodiment 2 of the present disclosure；
Fig. 4 is a schematic view showing determining a pose of a workpiece by a laser array according to Embodiment 1 and Embodiment 2 of the present disclosure； and
Fig. 5 is a flow chart of a method for detecting a narrow groove of a specular reflection workpiece according to embodiments of the present disclosure.
DETAILED DESCRIPTION
Reference will be made in detail to embodiments of the present disclosure. The embodiments described herein with reference to drawings are explanatory, illustrative, and used to generally understand the present disclosure. The embodiments shall not be construed to limit the present disclosure. The same or similar elements and the elements having same or similar functions are denoted by like reference numerals throughout the descriptions.
In the specification, unless specified or limited otherwise, relative terms such as “central” , “longitudinal” , “lateral” , “front” , “rear” , “right” , “left” , “inner” , “outer” , “lower” , “upper” , “horizontal” , “vertical” , “above” , “below” , “up” , “top” , “bottom” as well as derivative thereof (e.g. , “horizontally” , “downwardly” , “upwardly” , etc. ) should be construed to refer to the orientation as then described or as shown in the drawings under discussion. These relative terms are for convenience of description and do not require that the present disclosure be constructed or operated in a particular orientation.
In the description of the present disclosure, it should be understood that, unless specified or limited otherwise, the terms “mounted, ” “connected, ” and “coupled” and variations thereof are used broadly and encompass such as mechanical or electrical mountings, connections and  couplings, also can be inner mountings, connections and couplings of two components, and further can be direct and indirect mountings, connections, and couplings, which can be understood by those skilled in the art according to the detail embodiment of the present disclosure.
In addition, terms such as “first” and “second” are used herein for purposes of description and are not intended to indicate or imply relative importance or significance or to imply the number of indicated technical features. Thus, the feature defined with “first” and “second” may comprise one or more of this feature. In the description of the present invention, “aplurality of” means two or more than two, unless specified otherwise.
Fig. 1 is a schematic structural view of an apparatus for detecting a narrow groove of a specular reflection workpiece according to Embodiment 1 of the present disclosure.
As shown in Fig. 1, the apparatus includes a control unit 1, a spherical light source 3, a laser array 4 and an imaging unit 5. Each of the spherical light source 3, the laser array 4 and the imaging unit 5 has a fixed relative position with respect to a welding torch 8. The spherical light source 3 has a semispherical diffuse reflection body 32 and a light emitting diode array 31 arranged over the semispherical diffuse reflection body 32 and is configured to generate a uniform illumination on a workpiece surface 7. The laser array 4 has at least three lasers and is configured to emit a laser light onto the workpiece surface 7 so as to determine a pose of the welding torch 8 with respect to the workpiece surface 7. The imaging unit 5 is configured to sample a light reflected from the workpiece surface 7 and emitted from the spherical light source 3 or the laser array 4 so as to generate an image of the workpiece surface 7. The control unit 1 is electrically connected with the spherical light source 3, the laser array 4 and the imaging unit 5 respectively, and is configured to control the spherical light source 3 and the laser array 4 to illuminate the workpiece surface 7 alternately, to control the imaging unit 5 to sample a first image of the workpiece surface 7 illuminated by the laser array 4 and a second image of the workpiece surface illuminated by the spherical light source 3, to calculate the pose of the welding torch 8 with respect to the workpiece according to the first image, and to calculate a position of a center point of the narrow groove according to the second image and the pose of the welding torch 8 with respect to the workpiece.
In an embodiment of the present disclosure, the apparatus further includes a light filtering unit 6.The light filtering unit 6 is arranged in a light path between the spherical light source 3 and the  imaging unit 5 and is configured to filter an arc light generated during a welding process and an ambient light.
In one embodiment, the control unit 1 may be connected with the spherical light source 3, the laser array 4 and the imaging unit 5 via a wire.
In one embodiment, the spherical light source 3 further includes an aperture 33, through which the laser light emitted by the laser array 4 reaches the workpiece surface 7. Further, the light emitting diode array 31 is arranged on a bottom of the semispherical diffuse reflection body 32 and the light emitted by the light emitting diode array 31 reaches the workpiece surface 7 after being reflected by the semispherical diffuse reflection body 32. Moreover, a laser spot emitted from the laser array 4 is also projected onto the workpiece surface 7 and a light reflected from the workpiece surface 7 reaches the imaging unit 5 after passing through the aperture 33 and the light fileting unit 6. In other words, the light reflected from the workpiece surface 7 and emitted from the spherical light source 3 or the laser array 4 reaches the imaging unit 5 through the aperture 33.
It should be noted that, a space distribution of an output light intensity of the light emitting diode array 31 is seriously nonuniform, since the light emitting diodes are distributed sparsely and a light intensity direction of each light emitting diode is nonuniform. The inner surface of the semispherical diffuse reflection body 32 is equal to an Ulbricht sphere surface whose light intensity integration effect can eliminate the nonuniformity of the output light intensity of the light emitting diode array 31 to some extent, such that the light density projected onto the workpiece surface 7 is uniform. And compared with the laser source, the light emitting diode array 31 may provide an incoherent light having a uniform light density and without a disadvantage of speckle.
In embodiments of the present disclosure, the imaging unit 5 includes, but not limited to, any one of a charge-coupled device, a complementary metal oxide semiconductor imaging device, a position sensitive device and a charge-injection device. In addition, a wavelength of the light emitted by the spherical light source 3 or the laser array 4 is consistent with a central wavelength of the light filtering unit 6, and the central wavelength of the light filtering unit 6 is within a sensitive wavelength range of the imaging unit 5.
In an embodiment of the present disclosure, the number of the lasers is three, namely a first laser 41, a second laser 42 and a third laser 43 and a wavelength thereof is 635mm. The imaging unit 5 is a 1024×1024 CCD camera, and a field range thereof is 30mm×30mm and a detection  accuracy thereof is 0.03mm. A wavelength of the light emitted by the light emitting dioxide array 31 ranges from 635nm to 645nm. The light filtering unit 6 is a narrow bandpass filter whose central wavelength is 635nm and full width at half maximum is 10nm. Since the arc light has the relative weak light density at the wavelength range of 635nm-645nm, the light filtering unit 6 can filter the arc light generated during the welding process.
In an embodiment of the present disclosure, the apparatus further includes a housing 2. The welding torch 8 is fixedly connected to the housing 2, and the spherical light source 3, the laser array 4, the imaging unit 5 and the light filtering unit 6 are fixedly arranged in the housing 2 respectively.
Fig. 2 is a schematic structural view of an apparatus for detecting a narrow groove of a specular reflection workpiece according to Embodiment 2 of the present disclosure.
As shown in Fig. 2, Embodiment 2 differs from Embodiment 1 in that the light emitting dioxide array 31 is arranged on the inner surface of the semispherical diffuse reflection body 32. Thus, a part of the light emitted by the light emitting diode array 31 reaches the workpiece surface 7 directly and another part of the light emitted by the light emitting diode array 31 reaches the workpiece surface 7 after being reflected by the semispherical diffuse reflection body 32. The light density projected onto the workpiece surface 7 is uniform due to the light intensity integration effect of the semispherical diffuse reflection body 32.
Fig. 3 is an image of a groove sampled by an imaging unit when a spherical light source illuminates a workpiece according to Embodiment 1 and Embodiment 2 of the present disclosure.
As shown in Fig. 3, a base metal 71 and the groove 72 have great different optical reflection characteristics. A gray scale of the base metal 71 is nearly saturated due to a strong specular reflection on a surface of the base metal 71. The groove 72 is represented as a curve in the image, in which a gray scale of the groove 72 is close to zero, since the light projected onto the groove 72 is reflected by a side wall of the groove 72, and thus not reaching the imaging unit 5. Because of the great difference between the gray scales of the base metal 71 and the groove 72, the position of the groove 72 may be detected quickly and accurately.
Fig. 4 is a schematic view showing determining a pose of a workpiece by a laser array according to Embodiment 1 and Embodiment 2 of the present disclosure.
Since a spot of the light projected onto the workpiece surface 7 and emitted from the  spherical light source 3 has no evident profile feature, it is difficult to determine the pose of the welding torch 8 with respect to the workpiece surface 7. In the embodiment of the present disclosure, the laser array 4 is used to determine the pose of the welding torch 8 with respect to the workpiece surface 7 and a pixel coordinate system {P} is established in the image sampled by the imaging unit 5, in which any point in the pixel coordinate system {P} represents a pixel coordinate of the image sampled by the imaging unit 5. Moreover, an imaging unit coordinate system {C} is established, in which an origin of the imaging unit coordinate system {C} is an optical center of the imaging unit 5 and a direction of a vertical axis of the imaging unit coordinate system {C} is identical to that of an optical axis of the imaging unit 5.
Fig. 5 is a flow chart of a method for detecting a narrow groove of a specular reflection workpiece according to embodiments of the present disclosure.
When the control unit 1 controls the laser array 4 to turn on and the spherical light source 3 to turn off, the imaging unit 5 samples an image of the workpiece surface 7, i.e. , the imaging unit 5 samples the first image of a workpiece surface 7 illuminated by the laser array 4, so as to calculate the pose of the welding torch 8 with respect to the workpiece surface 7. Furthermore, the control unit 1 can also calculate a normal vector of the workpiece surface 7 according to the first image. When the controls unit 1 controls the spherical light source to turn on and the laser array 4 to turn off, the imaging unit 5 samples an image of the workpiece surface 7, i.e. , the imaging unit 5 samples the second image of the workpiece surface 7 illuminated by the spherical light source 3, such that a coordinate of a center point of the narrow groove 72 in the imaging unit coordinate system {C} can be calculated according to a pixel coordinate of the center point of the narrow groove 72 in the second image and a partial calculation result of the pose of the welding torch 8 with respect to the workpiece surface 7 (i.e. , the normal vector of the workpiece surface 7) . Specifically, the control unit 1 is configured to control the spherical light source 3, the laser array 4 and the imaging unit 5, to process the images sampled by the imaging unit 5 and to automatically adjust the pose of the welding torch 8 with respect to the workpiece surface 7, thus implementing an automatic tracking.
Accordingly, a method for detecting a narrow groove of a specular reflection workpiece includes following steps.
An imaging unit coordinate system {C} is established, in which an origin of the imaging unit  coordinate system {C} is an optical center of an imaging unit and a direction of a vertical axis of the imaging unit coordinate system {C} is identical to that of an optical axis of the imaging unit, and a pixel coordinate system {P} is established.
Given that a laser array has N lasers, in which N is an integer larger than or equal to 3, and a propagation path equation of a laser light emitted by an ith laser in the imaging unit coordinate system {C} can be obtained, which is denoted as Equation (1) :
Xi＝Xi,0+tiSi                      (1)
where i is an integer larger than or equal to 1 and less than or equal to N； Xi and Xi,0 are points in a propagation path of the laser light emitted by the ith laser； Si is a unit direction vector of the propagation path of the laser light emitted by the ith laser； ti is a directed distance between the points Xi and Xi,0.
The imaging unit is calibrated (for example, by a Zhang Zhengyou calibrating method) to obtain Xi,0 and Si in Equation (1) and a transformation relationship between a point (u, v)T in the pixel coordinate system {P} and a corresponding point (x, y, z)T in the imaging unit coordinate system {C} , which is denoted as Equation (2) :
Figure PCTCN2014086847-appb-000007
where f1 and f2 are transformation functions between the pixel coordinate system {P} and the imaging unit coordinate system {C} ；
The imaging unit samples a first image of a workpiece surface illuminated by the laser array. Then, the first image is processed to calculate a coordinate (ui, vi)T of a laser spot of the ith laser in the pixel coordinate system {P} and to calculate a coordinate Ai of the laser spot of the ith laser in imaging unit coordinate system {C} according to the coordinate (ui, vi)T.
Specifically, when the laser array is turned on and the spherical light source is turned off, the imaging unit samples the first image of the workpiece surface illuminated by the laser array. Given that a pixel coordinate of the laser spot projected onto the workpiece surface 7 and emitted from the ith laser in the first image sampled by the imaging unit is (ui, vi)T, and a point corresponding to (ui, vi)T in the imaging unit coordinate system {C} is Ai＝ (xi, yi, zi)T, which is obtained by Equation (3):
Ai＝ti,2 (ui, vi) Vi (ui, vi)                  (3)
where i is an integer larger than or equal to 1 and less than or equal to N, Vi (ui, vi) ＝ [f1 (ui, vi) , f2(ui, vi) , 1]T, and ti,2 (ui, vi) is an undetermined parameter depending on ui and vi. It is indicated in Equation (3) that the point Ai is in a straight line represented by Equation (3) , and the straight line passes the origin of the imaging unit coordinate system {C} and defines a direction vector Vi (ui, vi) .
Since the point Ai is in a straight line represented by Equation (1) , the point Ai may also be represented by Equation (4) :
Ai＝Xi,0+ti,1 (ui, vi) Si                 (4)
where ti,1 (ui, vi) is an undetermined parameter depending on ui and vi.
The point Ai is an intersection point of two straight lines represented by Equation (3) and Equation (4) , and the vectors Si and Vi (ui, vi) are not in parallel with each other, otherwise the point Ai will not exist. However, the straight lines represented by Equation (3) and Equation (4) are generally skewed lines due to a measurement error and an interference noise, and thus the point Ai is taken as a midpoint of a common perpendicular of the two skewed lines. Since a common perpendicular segment of the two skewed lines is a shortest one of line segments connecting two points respective in the two skewed lines, an objective function is established as:
Figure PCTCN2014086847-appb-000008
and ti,1 (ui, vi) and ti,2 (ui, vi) can be determined by solving a minimum value of the objective function g, such that the coordinate of the point Ai can be obtained.
The objective function g is processed as follows:
Assuming:
Figure PCTCN2014086847-appb-000009
i.e. ，
Figure PCTCN2014086847-appb-000010
A determinant of coefficient of equation set (7) is:
Figure PCTCN2014086847-appb-000011
where <Si, Vi (ui, vi) > represents an included angle of the vectors Si and Vi (ui, vi) .
Since the vectors Si and Vi (ui, vi) are not in parallel with each other, a determinant of equation (8) is larger than zero and equation set (7) has a unique solution which is denoted as:
Figure PCTCN2014086847-appb-000012
Figure PCTCN2014086847-appb-000013
Since the point Ai is the midpoint of the common perpendicular of the straight lines represented by Equation (3) and Equation (4) , and thus
Ai＝ [Xi,0+ti,1 (ui, vi) · Si+ti,2 (ui, vi) · Vi (ui, vi) ] /2              (11)
It can be proved that the intersection point coordinate still satisfies equation (11) , when the straight lines represented by Equation (3) and Equation (4) are not skewed lines.
So far, the coordinate Ai of the laser spot projected onto the workpiece surface and emitted from the ith laser in the imaging unit coordinate system {C} is given by Equation (11) . In an actual detection, all laser spots are ensured to be projected around the narrow groove, and assuming that the workpiece surface onto which the laser spot is projected is a plane denoted as W. An equation of the plane W is denoted as XTα＝1, where α is a normal vector of the plane W, and X is a point in the plane W. Since the point Ai is in the plane W, there is:
Figure PCTCN2014086847-appb-000014
i.e. ,
Figure PCTCN2014086847-appb-000015
Only if N is larger than or equal to 3, can equation (13) have a unique least square solution, and the equation of the plane W can be determined. Thus, the laser array is required to include at  least three lasers.
The imaging unit samples a second image of the workpiece surface illuminated by the spherical light source. A filtering and threshold segmentation process is performed on the second image, a coordinate (uw, vw)T of a center point of the narrow groove in the pixel coordinate system {P} is extracted, and a coordinate B of the center point of the narrow groove in the imaging unit coordinate system {C} is calculated according to the coordinate (uw, vw)T.
Specifically, when the spherical light source is turned on and the laser array is turned off, the imaging unit samples a gray scale image of the workpiece surface, i.e. , the second image. The filtering and threshold segmentation process is performed on the second image sampled by the imaging unit so as to obtain a binary image I. In the binary image I, the gray scale of a base metal is 1 and the gray scale of the narrow groove is 0. For a jth row of the second image sampled by the imaging unit, the coordinate (uw, vw)T of the center point of the narrow groove in the pixel coordinate system {P} is denoted as:
uw＝j                        (14)
Figure PCTCN2014086847-appb-000016
where I (j, v) represents a gray value of a pixel point in a jth row and a vth column of the image I； # {v: I(j, v) ＝ 0} represents a sum number of pixel points satisfying the equation: I (j, v) ＝ 0； and sum {v: I(j, v) ＝ 0} represents a sum of column numbers of pixel points satisfying the equation: I (j, v) ＝ 0.
Since a partial area of the workpiece surface can be approximately regarded as the plane W and the point B in the imaging unit coordinate system {C} corresponding to the point (uw, vw) in the pixel coordinate system {P} is in the plane W, according to Equation (2) and the equation of the plane W, there is:
B＝tw (uw, vw) Vw (uw, vw)                 (16)
BTα＝1                       (17)
where tw (uw, vw) is an undetermined parameter depending on uw and 
Figure PCTCN2014086847-appb-000017
and Vw(uw, vw) ＝ [f1 (uw, vw) , f2 (uw, vw) , 1]T (18) .
The coordinate B of the center point of the narrow groove in the imaging unit coordinate system {C} can be calculated according to equations (16) and (17) ,i.e.,
B＝Vw (uw, vw)/[αTVw (uw, vw) ] (19) .
A pose of a welding torch with respect to the workpiece surface can be adjust automatically according to the coordinate B of the center point of the narrow groove and the normal vector α of the workpiece surface, thus implementing an automatic identification and tracking of the narrow groove.
It should be noted that, the above embodiments are just intended to illustrate the present disclosure, but not to limit the present disclosure. For example, the laser array may include more than three lasers so as to improve the detection accuracy of the pose of the welding torch with respect to the workpiece surface； an imaging unit with a higher resolution may be used herein to improve the detection accuracy of the groove； and the light filtering unit may be replaced by a spectrometer (such as a monochrometer) .
The present disclosure uses the spherical light source, the laser array and the imaging unit to implement the detection of the narrow groove of the specular reflection workpiece. The apparatus and the method do not depend on macroscopic geometric structure properties of the groove and the detection accuracy is improved as high as 0.03mm. The spherical light source is used to generate the uniform illumination on the workpiece surface, such that the gray scale of the base metal is close to 1 and the gray scale of the groove is close to 0. Therefore, on one hand, the position of the groove in the image can be obtained accurately； on the other hand, a difficulty of processing the image and an algorithm complexity are reduced. Moreover, the laser array is used to determine the pose information of the workpiece surface, thus simplifying the method and calculating the pose deviation of the welding torch with respect to the workpiece surface quickly and accurately. The light filtering unit is used to filter the interference of the arc light and the ambient light on the imaging unit, such that the adaptability of the apparatus to an actual welding operation environment can be improved. In addition, the apparatus has a simple structure, a low cost and a good real-time performance, and applies to an automatic detection of the groove of the strong specular reflection workpiece, and particularly to a detection of the narrow groove having a groove gap less than 0.1mm.
Reference throughout this specification to “an embodiment, ” “some embodiments, ” “one embodiment” , “another example, ” “an example, ” “aspecific example, ” or “some examples, ” means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present  disclosure. Thus, the appearances of the phrases such as “in some embodiments, ” “in one embodiment” , “in an embodiment” , “in another example, ” “in an example, ” “in a specific example, ” or “in some examples, ” in various places throughout this specification are not necessarily referring to the same embodiment or example of the present disclosure. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.
Although explanatory embodiments have been shown and described, it would be appreciated by those skilled in the art that the above embodiments cannot be construed to limit the present disclosure, and changes, alternatives, and modifications can be made in the embodiments without departing from spirit, principles and scope of the present disclosure.

Claims (14)

  1. An apparatus for detecting a narrow groove of a specular reflection workpiece, comprising:
    a spherical light source having a semispherical diffuse reflection body and a light emitting diode array arranged over the semispherical diffuse reflection body, and configured to generate a uniform illumination on a workpiece surface；
    a laser array having at least three lasers and configured to emit a laser light onto the workpiece surface；
    an imaging unit configured to sample a light reflected from the workpiece surface and emitted from the spherical light source or the laser array so as to generate an image of the workpiece surface； and
    a control unit electrically connected with the spherical light source, the laser array and the imaging unit respectively, and configured to control the spherical light source and the laser array to illuminate the workpiece surface alternately, to control the imaging unit to sample a first image of the workpiece surface illuminated by the laser array and a second image of the workpiece surface illuminated by the spherical light source, to calculate a pose of a welding torch with respect to the workpiece according to the first image, and to calculate a position of a center point of the narrow groove according to the second image and the pose of the welding torch with respect to the workpiece.
  2. The apparatus of claim 1, wherein the spherical light source further comprises an aperture, through which the laser light emitted by the laser array reaches the workpiece surface.
  3. The apparatus of claim 2, wherein the light reflected from the workpiece surface and emitted from the spherical light source or the laser array reaches the imaging unit through the aperture.
  4. The apparatus of any one of claims 1-3, further comprising:
    a light filtering unit arranged in a light path between the spherical light source and the imaging unit and configured to filter an arc light generated during a welding process and an ambient light.
  5. The apparatus of any one of claims 1-4, wherein the light emitting diode array is arranged on a bottom of the semispherical diffuse reflection body and the light emitted by the light emitting diode array reaches the workpiece surface after being reflected by the semispherical diffuse  reflection body.
  6. The apparatus of any one of claims 1-4, wherein the light emitting diode array arranged on an inner surface of the semispherical diffuse reflection body, and a part of the light emitted by the light emitting diode array reaches the workpiece surface directly and another part of the light emitted by the light emitting diode array reaches the workpiece surface after being reflected by the semispherical diffuse reflection body.
  7. The apparatus of any one of claims 1-6, wherein the control unit is further configured to calculate a normal vector of the workpiece surface according to the first image.
  8. The apparatus of claim 7, wherein the pose of the welding torch with respect to the workpiece surface can be adjusted automatically according to the position of the center point of the narrow groove and the normal vector of the workpiece surface.
  9. The apparatus of any one of claims 1-8, wherein the imaging unit comprises any one of a charge-coupled device, a complementary metal oxide semiconductor imaging device, a position sensitive device and a charge-injection device.
  10. The apparatus of any one of claims 1-9, wherein a wavelength of the light emitted by the spherical light source or the laser array is consistent with a central wavelength of the light filtering unit, and the central wavelength of the light filtering unit is within a sensitive wavelength range of the imaging unit.
  11. The apparatus of any one of claims 1-10, further comprising a housing, wherein the welding torch is fixedly connected to the housing, and the spherical light source, the laser array, the imaging unit and the light filtering unit are fixedly arranged in the housing respectively.
  12. A method for detecting a narrow groove of a specular reflection workpiece, comprising:
    establishing an imaging unit coordinate system {C} , in which an origin of the imaging unit coordinate system {C} is an optical center of an imaging unit and a direction of a vertical axis of the imaging unit coordinate system {C} is identical to that of an optical axis of the imaging unit；
    establishing a pixel coordinate system {P} ；
    assuming that a laser array has N lasers, in which N is an integer larger than or equal to 3；
    calibrating the imaging unit to obtain a transformation relationship between a point (u, v)T in the pixel coordinate system {P} and a corresponding point (x, y, z)T in the imaging unit coordinate system {C} :
    Figure PCTCN2014086847-appb-100001
    and to obtain a propagation path equation of a laser light emitted by an ith laser in the imaging unit coordinate system {C} :
    Xi＝Xi,0+tiSi
    where f1 and f2 are transformation functions between the pixel coordinate system {P} and the imaging unit coordinate system {C} , i is an integer larger than or equal to 1 and less than or equal to N； Xi and Xi,0 are points in a propagation path of the laser light emitted by the ith laser； Si is a unit direction vector of the propagation path of the laser light emitted by the ith laser； ti is a directed distance between the points Xi and Xi,0；
    sampling a first image of a workpiece surface illuminated by the laser array via the imaging unit；
    processing the first image, calculating a coordinate (ui, vi)T of a laser spot of the ith laser in the pixel coordinate system {P} and calculating a coordinate Ai of the laser spot of the ith laser in imaging unit coordinate system {C} according to the coordinate (ui, vi)T:
    Ai＝ [Xi,0+ti,1 (ui, vi) · Si+ti,2 (ui, vi) · Vi (ui, vi) ] /2
    where
    Vi(ui, vi) ＝ [f1 (ui, vi) , f2 (ui, vi) , 1] T
    Figure PCTCN2014086847-appb-100002
    Figure PCTCN2014086847-appb-100003
    assuming that the workpiece surface where the laser spot is projected to a plane W, an equation of the plane W is denoted as XTα＝1, where α is a normal vector of the plane W, and X is a point in the plane W, provided the point Ai is in the plane W, there being:
    Figure PCTCN2014086847-appb-100004
    namely:
    Figure PCTCN2014086847-appb-100005
    solving the normal vector α of the plane W；
    sampling a second image of the workpiece surface illuminated by the spherical light source via the imaging unit；
    performing a filtering and threshold segmentation process on the second image, extracting a coordinate (uw, vw)T of a center point of the narrow groove in the pixel coordinate system {P} , and calculating a coordinate B of the center point of the narrow groove in the imaging unit coordinate system {C} according to the coordinate (uw, vw)T:
    B＝Vw (uw, vw)/[αTVw (uw, vw) ]
    where
    Vw(uw, vw) ＝ [f1 (uw, vw) , f2 (uw, vw) , 1]T.
  13. The method of claim 12, wherein the coordinate (uw, vw)T of the center point of the narrow groove in the pixel coordinate system {P} is denoted as:
    uw＝j
    Figure PCTCN2014086847-appb-100006
    where I (j, v) represents a gray value of a pixel point in a jth row and a vth column of an image I； # {v: I(j, v) ＝ 0} represents a sum of pixel points satisfying the equation: I (j, v) ＝ 0； and sum {v: I (j, v) ＝ 0} represents a sum of column numbers of pixel points satisfying the equation: I (j, v) ＝ 0.
  14. The method of claim 12 or 13, wherein a pose of a welding torch with respect to the workpiece can be adjusted automatically according to the coordinate B and the normal vector α of the workpiece surface.
PCT/CN2014/086847 2014-05-13 2014-09-18 Apparatus and method for detecting narrow groove of specular reflection workpiece WO2015172485A1 (en)

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